1. Technical Field of the Invention
The present invention relates to a high-speed roaming technology in the infrastructure type wireless LAN environment.
2. Related Art
Initially LAN (Local Area Network) is developed and utilized as the wire network. In recent years, the wireless LAN is developed and utilized in cooperation with the high communication speed and the progress of the mobile computing technology and the mobile terminal.
The wireless LAN is such a network system that a part of the wire LAN is constructed by the wireless system to incorporate the station into the LAN, and in many cases the Ethernet is employed as the wire LAN.
In this case, the station is normally the general term of PC having the FH-WLAN (Frequency Hopping-Wireless LAN) adaptor, and means the notebook PC into which the FH-WLAN card is inserted, the desktop PC which has the FH-WLAN•ISA (Industrial Reference Architecture) adaptor, or the like. In case the wireless LAN corresponds to the type that belongs to following {circle around (3)}, the station signifies the mobile station (terminal) that has the wireless terminal adaptor and exchanges the communication while moving.
As the wireless LAN, there are {circle around (1)} the wireless LANs which construct the closed network as the private network by them selves, {circle around (2)} the wireless LAN in which the access points (APs) to bridge the wireless terminal to the backbone network are provided as the network which is constructed to switch the terminal, that is connected to the existing wire LAN, to the wireless connection, and {circle around (3)} the infrastructure type wireless LAN in which a plurality of access points are provided to enable the communication with the mobile terminal such as the portable terminal, etc. which has the wireless terminal adaptor.
In the infrastructure type wireless LAN in the above {circle around (3)}, in order to execute the roaming (this means to execute the switching of the communication connection from one access point to another access point herein) from one access point into which the station is subscribed to another access point, the station executes the operation (scanning) to find the access point into which the station can subscribe after such station transmits the probe request frame and then receives the probe response frame.
The normal scan contains two type scans in the default (set state), i.e., the active mode scan and the passive mode scan. First the active mode scan is carried out, and then the scanning mode is switched into the passive mode scan if the access point cannot be found within BSS (Basic Service Set) and then the passive mode scan is carried out. In case the access point cannot be found yet, the active mode scan and the passive mode scan are repeated.
In the above operation (scanning operation), since the U.S. band (frequency band) employs 79 channels from 2400.0 MHz to 2483.5 MHz, for example, all channels (79 channels at maximum) must be scanned to find the connectable access point. The roaming is carried out by transmitting the authentication request frame (the frame by which the station requests the authentication to enter into the access point (IEEE802.11)) from the station to the access point, that is connected for the first time based on the scan, and then receiving the authentication response frame, by which the access point authenticates the station, from the access point.
However, according to the above roaming system, the mobile station (referred simply to as a “station” hereinafter) secedes from the present access point, then scans all connectable access points, and then performs the subscription procedure to the access point having the maximum RSSI (Receive Signal Strength Indicator). As a result, a total time required for the roaming is extremely extended from 1 second at minimum to 13 seconds at maximum. This means that the time to disconnect the communication with the station is expanded, and thus this puts the very serious problem to the AGV (Automatic Guided Vehicle), etc. that is desired to be always controlled since the station is moving.
Therefore, in the Japanese Patent Application No. Hei 11-268657, the applicant of this application has proposed the high-speed roaming method in which the roaming function is started after the beacon quality is reduced smaller than the threshold value, then their radio situations are compared with each other by scanning the access points that are registered as the neighboring access point, and then the subscription operation is carried out in synchronism with the access point having the best communication environment.
More particularly, in this high-speed roaming method, first it is checked by searching the memories whether or not the hopping information of the registered neighboring access points, that are saved and registered in the access point into which the station is currently subscribed, has been gotten (downloaded). In the case that the hopping information has been downloaded, when the beacon quality of the access point into which the station is currently subscribed (connected) is reduced below the threshold value, the hopping channel, the hopping pattern, and the TSF time are fitted to the neighboring access point via the wireless MAC controller based on the acquired hopping information, and then the probe request frame is transmitted to the directly neighboring access point to monitor the beacon RSSI.
The probe response frame that is sent from the access point in response to this request is received, then the access point having the highest RSSI out of the monitored RSSIs is decided as the roaming access point (i.e., the subscription access point), and then the authentication request frame is transmitted to the access point via the wireless MAC controller. When the authentication response frame is received from the access point, that has transmitted the authentication request frame, via the wireless MAC controller, the subscription access point is switched to the access point as the authentication request sending destination (=authenticator).
According to such operation, the station can download the hopping information of the neighboring access point from the access point to which the station is now connected (subscribed). Therefore, the access point having the highest RSSI is selected from the neighboring access points in a short time based on the hopping information that is downloaded when the beacon quality is lowered below the threshold value, and thus such access point can be decided as the roaming destination.
[Problems to be Solved]
However, according to the high-speed roaming method set forth in the Japanese Patent Application No. Hei 11-268657, before the operation is shifted to the subscription operation, the roaming function is started after the beacon quality is lowered below the predetermined threshold value, then the access points that have been registered as the neighboring access points are scanned, then their radio situations are compared mutually, and then the subscription operation is synchronized with the access point that has the best communication environment. As a result, there is the problem such that the time is consumed until the subscription operation.